In a method and apparatus for magnet resonance imaging, and rf excitation pulse is emitted for the excitation of spins in an object to be examined and simultaneously a slice-selection gradient is activated during the rf excitation pulse, a pulse series of refocusing pulses is emitted, a slice-selection gradient pulse is activated during each refocusing pulse, and alternating readout gradient pulse series are activated, each series being in the pulse between two refocusing pulses, with a de-phasing pulse being activated immediately before each readout gradient pulse series and a re-phasing pulse being activated immediately after the each readout gradient pulse series such that a corresponding offset is produced in the readout direction so that each readout gradient pulse series scans a part of the k-matrix in the kx direction in a serpentine-like manner, with the serpentine-like scanning of each partial section being implemented by activating short phase-coding gradient pulses during each zero point of the corresponding readout-gradient pulse series.
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1. A method for magnetic resonance imaging comprising the steps of:
(a) emitting a radio-frequency excitation pulse for exciting nuclear spins in an object, the excited nuclear spins causing magnetic resonance signals to be emitted;
(b) selecting a slice in said object in which said nuclear spins are excited by simultaneously activating a slice selection gradient during emission of said radio-frequency excitation pulse;
(c) activating a pulse series comprising a plurality of refocusing pulses;
(d) during each refocusing pulse, activating a slice selection gradient pulse; and
(e) activating alternating readout gradient pulse series respectively between successive ones of said refocusing pulses, and activating a dephasing pulse immediately before each readout gradient pulse series and activating a re-phasing pulse immediately after readout gradient pulse series for producing an offset in a readout direction for causing each readout gradient pulse series to scan a portion of k-space in a kx direction in a serpentine-like path, said serpentine-like path in each part of k-space being produced by activating short phase-coding gradient pulses during each zero point of the associated readout gradient pulse series.
13. A magnetic resonance tomography apparatus comprising:
a sequence controller;
a) emitting a radio-frequency excitation pulse for exciting nuclear spins in an object, the excited nuclear spins causing magnetic resonance signals to be emitted, and for receiving said magnetic resonance signals;
a gradient system for simultaneously activating a slice selection gradient during emission of said radio-frequency excitation pulse for selecting a slice in said object in which said nuclear spins are excited;
said sequence controller activating a pulse series from said rf antenna comprising a plurality of refocusing pulses and during each refocusing pulse, activating a slice selection gradient pulse from said gradient system; and
said sequence controller activating alternating readout gradient pulse series from said gradient system respectively between successive ones of said refocusing pulses, and activating a dephasing pulse from said gradient system immediately before each readout gradient pulse series and activating a re-phasing pulse from said gradient system immediately after readout gradient pulse series for producing an offset in a readout direction for causing each readout gradient pulse series to scan a portion of k-space in a kx direction in a serpentine-like path, said serpentine-like path in each part of k-space being produced by activating short phase-coding gradient pulses during each zero point of the associated readout gradient pulse series.
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1. Field of the Invention
The present invention concerns, in general, the magnetic resonance tomography (MRT) as used in medicine to examine patients. The present invention concerns, in particular, an MRT imaging procedure for the acquisition of high-resolution single-shot images in a short scanning period.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear spin resonance and has been successfully employed in medicine and biophysics as an imaging procedure for more than 15 years. In this modality, the object is exposed to a strong, constant magnetic field. The nuclear spins of the atoms, which were previously randomly oriented, thereby are aligned in the object. Radio-frequency energy can then excite these “ordered” nuclear spins to a specific oscillation. This oscillation creates the actual measurement signal in MRT, which is recorded using suitable receiving coils. By the use of inhomogeneous magnetic fields, generated by gradient coils, the examination subject, and the signals therefrom, be encoded spatially in all three spatial directions, which is in general called “spatial encoding.”
The acquisition of the data in MRT takes places in k-space (frequency domain). The MRT image in the image domain is linked with the MRT data in k-space by means of Fourier transformation. The spatial encoding of the object, which spans k-space, takes place by means of gradients in all three spatial directions. A distinction is made between the slice readout gradient (determines an absorption slice in the object, usually along the z-axis), the frequency coding gradient (determines a direction in the slice, normally along the x-axis), and the phase-coding gradient (determines the second dimension within the slice, usually the y-axis).
First, a slice of the subject, e.g. in the z direction is selectively stimulated. The coding of the location information in the slice takes place by a combined phase and frequency coding by means of these two already-mentioned orthogonal gradient fields that in the example of a slice stimulated in the z-direction, are generated by the aforementioned gradient coils in the x and y-directions, respectively.
A first possible sequence for recording the data in an MRT scan is shown in
In order to measure an entire layer of the object to be examined, the imaging sequence is repeated N times for different values of the phase encoding gradient e.g. Gy, with the frequency of the magnetic resonance signal (spin-echo signal) being scanned, digitized, and stored N times in equidistant time intervals Δt in the presence of the readout gradient for each sequence execution using a Δt-clocked ADC (Analog Digital Converter). In this manner, a line-by-line numerical matrix (matrix in k-space or k-matrix) with N×N data points is obtained in accordance with
The scanning of the k matrix (or k matrices in the case of data acquisition from several layers) for spin echo sequences with diagnostically usable image quality normally requires several minutes of measurement time, was can be a problem for many clinical application. For example, patients cannot remain immobile for the required period of time. For examinations of the thorax or in the pelvic region movement of the anatomy is generally unavoidable (cardiac and respiratory movements, peristalsis). One way to accelerate the spin echo sequence was published in 1986 as the Turbo Spin Echo sequence (TSE sequence) or under the acronym RARE (Rapid Acquisition with Relaxation Enhancement) (J. Hennig et al. Magn. Reson. Med. 3, 823-833, 1986). In this procedure, that is much faster compared to the conventional aforementioned spin echo procedure, several multiple echoes are created based on a 90° excitation pulse, with each of these echoes being individually phase-encoded. A corresponding sequence diagram is shown in
An even faster imaging sequence is a combination of RARE with the half-Fourier technique that was introduced in 1994 as the so-called HASTE sequence (Half-Fourier Acquired Single-Shot Turbo Spin Echo) (B. Kiefer et al., J.Magn. Reson. Imaging, 4(P), 86, 1994). HASTE uses the same basic technique as RARE, but only half of the k-matrix is scanned. The other half of the k-matrix is reconstructed by calculation manner using a half-Fourier algorithm. For this purpose, use is made of the fact that the data points of the k-matrix are arranged mirror-symmetrical to the mid-point of the k-matrix. Thus, it suffices to only measure the data points of one k-matrix half and to complete the raw data matrix by mirroring with respect to the mid-point (and complex conjugation). In this manner, the measurement time can be reduced by half. The reduction of the recording time, however, is has an associated degradation of the signal to noise ratio (S/R) by a factor of √42.
A further method for quickly obtaining and scanning the k-matrix is the “echo-planar imaging” (EPI) procedure. The main idea behind this procedure is to generate a series of echoes in the readout gradient (Gx), which are assigned to different lines in the k-matrix through a suitable gradient switch (modulation of the phase encoding gradient Gy), after a single (selective) RF stimulus in a very short period of time. In this manner, all lines of the k-matrix can be acquired with one single sequence execution. Different versions of the method also known as the echo-planar technique differ only as to how the phase-encoding gradients are switched, i.e. how the data points of the k-matrix are scanned.
The ideal form of an echo-planar pulse sequence is shown in
The scanning of the echo sequence must be completed in a time that corresponds with the dismantling of the transverse magnetization T2*. Otherwise, the different lines of the k-matrix would namely be weighted according to the sequential order of their acquisition: certain location frequencies would be overemphasized and others underemphasized.
Another quick MRT imaging procedure is GRASE (Gradient and Spin-Echo) GRASE was first introduced in 1995 (D. Feinberg et al. Magnetic Resonance Medicine 33, 529-533, 1995) and can be seen as a hybrid technique of EPI and RARE with the underlying idea of scanning the spin-echo envelope with several gradient echoes. With GRASE, as shown in
An object of the present invention is to provide an imaging procedure for magnetic resonance tomography that has improvements over the imaging procedures described above. The improvements include, among other things, a decrease in the susceptibility artifacts and a lower sensitivity with respect to dynamic field changes compared to EPI as well as a less complex signal modulation in k-space compared to GRASE, which then leads to fewer ring and ghost artifacts. Furthermore, a lower specific absorption rate (SAR) is described as well as fewer artifacts with respect to movement-induced phase shifts with diffusion-weighted imaging.
This object is achieved in accordance with the invention in an MRT method and apparatus wherein a radio-frequency excitation pulse is emitted for the excitation of spins in an object to be examined, a slice-selection gradient (GS) is simultaneously activated during the radio-frequency excitation pulse, a pulse series of refocusing pulses is generated, a slice-selection gradient pulse is activated during each refocusing pulse, and a series of alternating readout gradient pulses is activated in each pause between two refocusing pulses, with a de-phasing pulse being activated immediately before each readout gradient pulse series and a re-phasing pulse being activated immediately after each readout gradient pulse series such that a corresponding offset is created in the readout direction so that each readout gradient pulse series scans a part of the k-matrix in the kx direction in a serpentine-like manner, with the serpentine-like scanning of each partial section being implemented by activation of short phase-coding gradient pulses (GP) during each zero point of the corresponding readout-gradient pulse series.
The radio-frequency excitation pulse is preferably a 90° pulse. Each refocusing pulse is also preferably a 180° pulse, and the refocusing pulses also preferably have the same chronological spacing from one another.
The gradient moment (the chronological time integral over the gradient amplitude) of each readout gradient pulse string is preferably so small that a correspondingly short gradient echo spacing can be selected such that the procedure is insensitive with respect to magnetic field susceptibilities as well as dynamic magnetic field continuities.
Moreover, the procedure according to the invention can be combined with a partial Fourier acquisition technique, with a partial parallel imaging technique (SMASH, SENSE, GRAPPA) and/or with a magnetization preparation using RF pulses and magnetic field gradient pulses (diffusion weighting, fluid suppression).
Furthermore, in a first preferred embodiment of the procedure in accordance with the invention, only a portion of the k-matrix in the kx and/or ky direction(s) is scanned after a radio-frequency excitation pulse and the scanning of the entire k-matrix is realized by the progression of several radio-frequency excitation pulses.
In a second preferred embodiment of the procedure in accordance with the invention, the procedure is used in combination with phase coding in the slice selection direction using slice-selective or non-slice-selective excitation pulses for 3D imaging.
The magnetic resonance tomography apparatus according to the invention has gradient amplifiers with respectively associated gradient coils, a display terminal a sequence controller and system processor as well as an analog-to-digital converter (ADC) that is suitable for performing the procedure described above.
A cylindrical gradient system 3 formed by three coils is disposed in the basic field magnets 1. An amplifier 14 supplies each coil with power for the generation of a linear gradient field in the respective directions of the Cartesian coordinate system. The first coil of the gradient system 3 generates a gradient Gx in the x direction, the second coil a gradient Gy in the y direction, and the third coil a gradient Gz in the z direction. Each amplifier 14 has an analog-to-digital converter, which is controlled by a sequence controller 18 for the correct chronological activation of the gradient impulses.
An RF antenna 4, which converts the signal pulse provided by an RF power amplifier 30 into a magnetic alternating field for the stimulation of the nuclei and the alignment of the nuclear spin of the object to be examined or the area of the object to be examined, is located within the gradient system 3. The RF antenna 4 also converts the alternating field emitted by the nuclear spins, i.e. generally the spin echoes brought about by a pulse sequence having one or more RF pulses and one or more gradient pulses, into a voltage that is fed to a high-frequency reception channel 8 of an RF system 22 via an amplifier 7. The RF system 22 has a transmission channel 9, in which the RF pulses for the stimulation of magnetic resonance are generated. The corresponding RF pulses are represented digitally as a pulse sequence determined by the system processor 20 in the sequence controller 18, as a series of complex numbers. This numerical series (sequence) is supplied as a real part and an imaginary part via inputs 12 to a digital-to-analog converter in the high-frequency system 22 and from this to the transmission channel 9. In the transmission channel 9, the pulse sequences are modulated up to an RF carrier signal, the base frequency of which corresponds to the resonant frequency of the nuclear spins in the measurement volume.
Switching between the transmission and reception modes takes place via a diplexer switch 6. The RF antenna 4 emits the RF pulse for the excitation of the nuclear spins into the measurement volume M and detects the resulting echo signals. The detected (received) magnetic resonance signals are demodulated in a phase-sensitive manner in the reception channel 8 of the RF system 22 and are converted via respective analog-to-digital converters for the real part and the imaginary part of the MR signal. An image is reconstructed by an image computer 17 from the data obtained in such a way. Management of the MR data, the image data, and the control programs take places via the system processor 20. The sequence controller 18 controls the generation of desired pulse sequences and the corresponding scanning of k-space, using a default with control programs. In particular, the sequence controller 18 controls the chronologically correct switching of the gradients, the emission of the RF pulses with a defined phase and amplitude as well as the receipt of the MR signals. The time base for the RF system 22 and the sequence controller 18 is made available by a synthesizer 19. The selection of suitable control programs for the generating of an MR image as well as the representation of the MR image takes place via a terminal 21, which has a keyboard as well as one or more monitors.
The imaging sequence in accordance with the invention is generated in particular in the sequence controller 18 or in the system processor 20. The sequence diagram of the sequence in accordance with the invention is shown in
The k-space trajectory or the scan behavior of this MRT imaging sequence in accordance with the invention is shown in
In summary, the MRT imaging sequence in accordance with the invention is based on scanning the spin echo created by the 180° refocusing pulse during a given readout gradient pulse of a defined length (called component pulse series above) in the form of gradient echoes, with only part of the k-matrix being scanned. The scanning of the entire k-matrix takes place by a linking or stringing together of additional 180° refocusing pulses with the corresponding phase-coded (blipped) readout gradient pulses. The scanning of different x regions of matrix in the k-space (segmentation in the readout direction) makes the following sequence designation meaningful: “spatial encoding using readout segmentation.” The acronym SPURS will be used below.
SPURS represents an equivalent alternative to known MRT imaging sequences but with significant advantages, in particular over the aforementioned fast MRT imaging procedures (TSE, RARE, HASTE, GRASE):
1. The number of required RF pulses (90° pulse with the ensuing 180° refocusing pulses) in SPURS is, with a consistent image quality, much lower than for procedures that have equivalent acquisition time. Thus, 128 RF pulses are required for a matrix size of 128×128 voxel in the RARE or HASTE radiographic techniques, approx. 25 for GRASE, and approx. 7 for SPURS. Fewer RF pulses results in the fact that slice-selective refocusing pulses can or may be used to a higher degree and thus no recognizable time delays occur between the absorption of several layers. Thus, the acquisition period for 20 slices takes a total of approx. 5 seconds compared to measurement with non-selective refocusing pulses, which takes more than a minute for the same number of slices (non-selective refocusing pulses lead to the fact that a break of several seconds is necessary between measurements of different slices for the regeneration of the magnetic resonance signal). Additionally, the low number of RF pulses in SPURS compared to RARE, HASTE, and GRASE results in a clearly reduced SAR, which means an advantage for body imaging, in particular with higher magnetic field strengths (for example, 3 Tesla), where SAR limitations represent a limiting factor. Furthermore, a low number of RF pulses has a particularly positive effect for diffusion-weighted imaging.
In diffusion-weighted imaging, de- and re-phasing gradients (as preparation gradients) are applied or switched before the start of the actual data acquisition. Even slight patient movement during the preparation period can cause phase changes in the signal and thus nullify the so-called CMPG (Carr Purcell Meiboom Gill) condition. This condition is fulfilled when the excitation pulse—in comparison to the subsequent refocusing pulses—has a 90° phase shift, but is no longer fulfilled when a non-reversible phase change occurs between RF excitation and the first refocusing pulse. A non-reversible phase change is a phase change that cannot be reversed by the refocusing pulse (echo-like) (e.g. a phase change induced by movement occurring in diffusion sequences). A reversible phase change is e.g. a phase evolution caused by resonance offset. A violation of the CPMG condition and resulting artifacts occur less often the fewer RF or refocusing pulses are used.
2. SPURS requires a high gradient switching rate (slew rate) with a comparably low gradient amplitude (due to the narrow scan area in the readout direction). This means that SPURS is particularly suitable for use in whole-body gradient coils, since normally the total theoretical efficiency of whole-body coils cannot be completely used due to the normally required high gradient amplitudes that reduce the stimulation limits.
3. Since each readout gradient pulse series only serves to scan a portion in the kx direction, only a relatively low gradient moment is required, so a correspondingly short gradient echo spacing is enabled. A short gradient echo spacing, however, causes a readout of the resonant signal preferably in the middle area of the respective refocusing pulse, which is why an accumulation of disturbance phases is avoided based on magnet field susceptibilities and the occurrence of susceptibility artifacts is prevented. Furthermore, a short gradient echo spacing causes the lower sensitivity of the SPURS imaging sequence compared to dynamic magnet field discontinuities that are caused, for example, by eddy currents.
4. In comparison to GRASE, in which a complex signal modulation occurs in the ky direction due to T2 relaxation, T2* relaxation, as well as an off-resonant phase development, data acquisition of all ky scan points for a corresponding spin echo per SPURS leads to a smoother version of the raw data signals in k-space. For this reason, ghost and ring artifacts are avoided with SPURS in contrast to GRASE.
5. All central k-space points are acquired with the same spin echo, which leads to a better contrast behavior (compared to a multi-shot TSE sequence).
It is further noted that SPURS can be combined advantageously with other MRT-imaging procedures, for example with
a) the partial Fourier acquisition technique (P.Margosian, Abstract in proc. SMRM, p. 1024, 1985), in which data from one or more kx segments are only acquired on one k-space half,
b) a standard-partial parallel imaging technique (e.g. SMASH D. Sodickson Magn. Reson. Med. 38, p. 591-603, 1997—SENSE K. Pruessman, Magn. Reson. Med. 42, p. 952-962, 1999 or GRAPPA M. Griswold, Magn. Reson. Med. 47, p. 1202-1210, 2002) in order to reduce the number of measured ky scan points (and thus the spin echo spacing) using several RF receive coils, and with
c) a magnetization preparation using RF pulses and magnetic field gradient pulses for improvement of the image contrast (examples: diffusion weighting, fluid suppression).
In a preferred version of the procedure in accordance with the invention, only a part of the k-matrix is scanned in the kx or ky direction after a high-frequency excitation pulse, and the scanning of the entire k-matrix is achieved by the progression of several RF excitation pulses. A procedure modified in this manner represents a multi-shot technique, analogous to the manner in which a single-shot RARE sequence is modified to a turbo-spin-echo sequence (TSE sequence).
In another preferred version of the procedure in accordance with the invention, the procedure is used in combination with phase coding in the slice-selection direction using slice-selective or non-slice-selective excitation pulses for 3D imaging.
Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.
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